Wednesday, May 2, 2012

Dinosaurs with ouchies on their noggins

A pair of Pachycephalosaurus put their heads together at Drayton Manor Theme Park. Photo by Matthew Wells, via Flickr.

Since I was a kid, one image has been stuck in my head when I think of the pachycephalosaurs, those rugged ornithischians with the outrageously thick frontoparietal bone: two males ramming into each other headlong like the two dudes in the photo above. I'm pretty sure I'm not alone in this. Even when I read a piece expressing skepticism about the behavior, I think to myself, "okay, but we're going to stick with the rule of coolness here." It's the same attitude that annoys me when people choose to not believe that many theropods wore feathery integument of some fashion. Again, I remind myself that science isn't about accepting things because they seem to make sense or they're pleasant to believe. So it's good that he head-butting hypothesis has recently had a spate of research devoted to it. The latest is a new paper in PLoS ONE, Cranial Pathologies in a Specimen of Pachycephalosaurus.

The rock has offered us scant remains to work from, which is probably a good reason why the pachycephalosaurs - who Tom Holtz refers to lovingly as "buttheads" in his Dinosaurs encyclopedia- have long been the subject of debate. Why were those heads so thickened? How many taxa might simply represent different growth stages of one species? Could they sustain direct head-to-head impacts or was it more likely that they smacked each other sidelong? Because of the patchiness of their fossil record, it's been difficult to put many of these ideas to the test.

In this new study, University of Wisconsin paleontologist Joseph Peterson, with the help of Christopher Vittore from the Department of Radiology at Rockford Memorial Hospital in Rockford, IL employ CT scanning of a Pachycephalosaurus frontoparietal dome at the Burpee Museum of Natural History and compare its apparent signs of injury to lesions in modern animals. Are these spots best interpreted as sites of bone resorption, as the rigors of decay, fossilization, and erosion, or are they evidence of traumatic impact?

Peterson's study concludes that it ain't the first two. Bone resorption, studied extensively in chasmosaurine fossils, leaves a smooth surface, as opposed to the roughness of the lesions in the studied Pachycephalosaurus dome. Signs of likely taphonomic suspects - such as scavenging, insect traces, and water wear - are also absent. But the lesions do match the marks left on the the crania of modern birds which have sustained impact and lived to tell the tale. Moreover, Peterson writes that the marks "are clustered over the thickest region of the dome; this distribution corresponds well to the location of expected traumatic pathologies resulting from agonistic behavior proposed for pachycephalosaurids." That would be head-butting, though they could have been smacking into boulders instead, I suppose. It's less absurd than some recent dinosaur stories in the popular press.

This follows another recent study by Eric Snively and Jessica Theodor, who compared the skull of Stegoceras to modern "buttheads" and found that "some pachycephalosaurs were as competent at head-to-head impacts as extant analogs displaying such combat," including musk oxen and duikers. (check out Eric's guest post at Archosaur Musings for more on this). On Facebook, Andy Farke joked that the "P" in PLoS ONE may as well stand for "pachycephalosaur." That wouldn't be so bad, would it?

Also be sure to follow Dr. Peterson on Twitter and at his blog (which has been dutifully added to the LITC blogroll).

You'd have thought that if the skulls were just for display that they would not be constructed out of thick, dense bone. They were smacking into something and it's more likely to be each other than predators or rocks and trees.

I recall reading earlier that they couldn't have been head-butting (other heads) because their spherical heads would glance off each other. That might be the case if they were bowling balls but seems to ignore any integument overlying the bone. Besides, you can actually smash two bowling balls together with a decent amount of force and not have them glance off each other even if you don't hit them dead centre. I'm sure that with some practice the Pachys would have worked it out.

I'm not sure if that "glancing off" idea was the result of actual testing or just an idea thar was stated. Ralph Chapman did a computer model that suggested head-to-head without glancing off wad possible; I may need to see if I can find both of these. At any rate the work by Peterson, Snively, and others recently is strong support for headbutting.

To expand a bit, the highly variable extremes of doming within pachycephalosaurids is similar to the highly variable parietosquamosal shield and horn morphologies in ceratopsians, which in turn is also similar to the variation we see in horn morphologies in extant bovids.

Farke et al, 2009 showed statistically significant differences in the distribution of cranial injuries in centrosaurines and chasmosaurines (Centrosaurus and Triceratops), suggesting that the differences in shield and horn morphologies are due to differences in agonistic combat styles. This same correlation is seen in modern horned mammals; Bighorn Sheep, with their robust curved horns actually do hit head-to-head with tremendous force. Alternatively, Mountain Goats, with their short, stubby, sharp horns, actually flank-butt each other. To add a third to the mix, Bison head-shove each other rather than get a running start. All in all, differences in horns, shields, etc, imply different styles of agonsitic combat.

So, look at something like Pachycephalosaurus, with its large, high-angled robust dome and compare it to something like Hansseusia, which has a smaller, relatively lower-angled dome. Add in the variation in squamosal horn clusters and you have completely different sets of tools for intraspecific combat. Maybe the little guys like Stegoceras or Hansseusia were actually going head-to-head or head-to-flank, while Pachycephalosaurus was behaving more like a Bison...or vice versa.

To throw in another wrench, if the Pachycephalosaurus-Stygimoloch-Dracorex ontogeny thingie is true, then perhaps we are seeing behavioral ontogeny as well!

Jeez, thanks for stopping by and leaving such a great comment. Not much I can add there. Thanks for adding to the literature on pachycephalosaurs - one of my favorite mesozoic daydreams (I'm that nerdy, yeah), is to imagine witnessing pachys ramming each other. Just imagine the sounds...

HA! I was right on the zeitgeist with my previous post then! I'm so happy that "buttheads" get to actually Butt Heads again instead of nuzzling each other gently in the flanks like a bunch of weak-necked pansy-arsed bone-for-brains.

Hear, hear, Hadiaz. I meant to remark on that point earlier, but forgot. Not clashing domes does not negate the force of the blow. Had pachycephalosaurs been butting flanks instead of heads, the impact might have been rib-shattering.

Hi, I'm fascinated by these creatures. I have two thoughts: 1): Could they have been simply using their heads to push against each other in contests to strength to win mates or dominance? This might not leave much evidence of trauma. I've seen this behavior in African buffalo, where they are not so much butting heads as pushing against each other. 2): Could their massively thick skulls have also had a keratin covering? Again, I am thinking of African buffalo with their massively thick horns. This would provide a surface that could stand battering which might not show up on skull bones.